Electric resistance element



Patented Dec. 4, 1923.

* TED FRANK A. FAHRENWALD, OF CLEVELAND HEIGHTS, OHIO.

ELECTRIC RESISTANCE ELEMENT.

No Drawing.

To all whom it may concern:

Be it known that I, FRANK A. FAHREN- WALD, a citizen of the UnitedStates, residing at Cleveland Heights, in the county of Cuyahoga andState of Ohio, have invented a certain new and useful Improvement inElectric Resistance Elements, of which the following is a full, clear,and exact description, reference being had to the accompanying drawings.v

This invention relates to electrical resistance elements and has for itsobject the provision of a new alloy for the same which shall possess ahigh and uniform co-efficient of resistance coupled with a high degreeof resistivity to oxidation or other corrosive in.- fluences encounteredin the use of such devices. Another object of the invention is therovision .of an alloy which shall be readlly fabricated into thenecessary physical forms, which shall possess -a great rigidity of shapeat high temperatures, which shall not become brittle, soft or short uponheating either continuously or intermittently, which shall posscssthesame resistance at all times and which can be cheaply roduoed frommaterials of low intrinsic va ue; while further objects and advantagesof the invention will become apparent asthe description proceeds.

I employ iron as the base of my improved alloy because of itsavailability and cheapness; with it I alloy chromium for the purpose ofincreasing its melting point, and its resistance to oxidation and othercorrosion, and manganese for the purpose of overcoming its hardening ortempering qualities as wel as increasing the specific electricalresistance. Carbon and silicon are present as impurities but each toless than one-half of one percent, and preferably even less than that.

-It is well known that electrical resistances are exposed to widetemperature variations, in fact some times even being chilled orquenched as when a cooking element is splashed with water. If theelement is made ofv an alloy which possesses hardening or temperingqualities, such as are characteristic of the steels, such treamentalways tends to ,embrittle it and also causes resistance fluctuation;the resistance becomes suddenly increased upon such hardening, de-

creasing the temperature subsequently pro-- curable with a fixed voltageuntil after a Application filed February 6, 1922. Serial No. 534,345.

' sufiicient length of time has elapsed for a complete'annealing. Insome cases the resistance is so much increased by the chilling thatreheating of the element is very slow and the annealing likewiseprolonged.

Of course it is the carbon present with the ferrous metal which producesthis condition, and owing to the unavoidable presence of carbon as animpurity this condition is always present unless the peculiar hardeningand tempering properties of. the carbon be overcome by the addition ofsome other element of suitable nature and quantity. The range of choiceis restricted by the considerations that the alloy must possess a highmelting point, be non-corrodible, be malleable and ductile at alltemperatures, stiff and strong at'high temperatures, and possess ahighand uniform co-eific-ient of electrical resistance. The only alloyswhich can fulfil these conditions are those which exhibit a solidsolution condition throughout the entire range of working temperatures.

Chromium has the properties, when employed in suflicient quantity, ofovercoming the hardening and tempering effect of carbon,'but to producethis-result by' the addition of chromium alone requires upwards oftwenty percent of chromium, indeed twenty percent is hardly suflicient;but the ness and lack of ductility of the alloy. The

main reason for this condition is doubtless the presence of carbon as animpurity, since it is even more difiicult to exclude carbon fromchromium alloys than from iron alloys. It has been suggested 'to replacea portion of the chromium with nickel, since it is found that two partsof nickel has approximately the same effect in overcoming the hardeningeffect of carbon as one part of chromium.

Nickel is further advantageous in possess ing a higher resistance tooxidation than iron and a smaller afiinity for carbon than either ironor chromium, and it, produces solid solution alloys of satisfactoryphysical properties when cold, although somewhat inclined to 10p and sagwhen hot. "The main drawback to the nickel alloy is its necessity ofreadjusting the proportion of chromium.

rise to other grave difficulties,

to iron at a ratio which is higher than that which occurs in nature.

It is also knownthat silicon, when alloyed with iron will overcome thehardening and tempering effects of carbon, but this property of siliconis not available for electrical resistance purposes owing to the factthat it renders the material hot-short when present in amounts greaterthan about .5%. This is too small an amount to modify the carbon-qualityperceptibly, since the effect of silicon is only about one fourth thatof a like weight of chromium. is

T have discovered that manganese can be employed in connection with thechromium to overcome the carbon effect in producing qliiiench-hardeningof ferrous metal and with t e production of an electrical resistanceelement possessing every one of the properties above enumerated asessential. It is I true that manganese tends to lower the melting pointand is, to this extent, a disadvantage, but its efi'ect in overcomingquenchhardening is approximately twice that of an equal weight ofchromium, and its addition to iron, even in small quantities, has theresult of considerably increasing the specific electric resistance.Accordingly I prefer to) restrict the manganese to about 3% or less ofthe alloy. This enables me to decrease the chromium content to twentyper cent or less, thereby greatly facilitating the manufacture ofthealloy, it enables the complete omission of nickel, which materiallycheapens it, and it renders the alloy more ductile at low temperaturesand more rigid at high temperatures, without decreasing the meltingpoint to such an extent as to interfere with the practical use of thealloy. This quality, however, restricts the proportion which may beused.

I have had excellent. results by the use of an alloy- Per cent. Chromium18 Manganese 2% Silicon .3 Carbon .2 Iron 79 The best range of myimproved alloy for electrical resistance purposes appears to be Theforegoing can be considered either the furnace charge or the ultimatecomposition of the alloy since if worked in a competent manner in anelectric furnace the loss is negligible.

Ingots of such alloy can be forged into billets, rolled into rods, anddrawn into wire without cracks or seams. The first drawing can beeffected hot, but after some working it can be drawn cold although thetensile strength is such as to render it desirable to elevate thetemperature at all stages. Elements made therefrom can be heated even to1800 F. and quenched without becoming hard or brittle or exhibiting anymaterial change in resistance. The temperature coeflicient is positiveand low. The wire is strong at all temperatures and even at 1800 F.,does not sag when supported only at considerable intervals. It can bemade from ordinary commercial ferrochrome and low carbon iron.

Having thus described my invention What I claim is:-

1. An electrical resistance element made of an alloy of iron from about16% to about 20% of chromium and about 1% to about 3% of manganese.

2. An electrical resistance element made of an alloy of iron with fromabout 16% to about 20% of chromium, and about 1% to about 3% ofmanganese, carbon and silicon being restricted to less than .5% each.

3. An electrical resistance element made of an alloy containing betweenabout 16% to 20% of chromium and between about 1% and 3% of manganese,and the balance mostly iron.

. In testimony whereof, I hereunto aflix my signature.

- FRANK A. FAHRENWALD.

